In the case of fluid-cooled nuclear reactors, problems associated with fuel bundle damage have underlined the need to have a capability to obtain accurate measurement data directly from within any fuel channel during any phase of reactor operation. The inability to obtain structural integrity measurement data directly from locations within fuel channels during reactor operation has resulted in uncertainty as to quantifying activity within the fuel channels, such activity relating more particularly to the pressure tubes, coolant flow and fuel bundles. This uncertainty has in turn resulted in severe information constraints regarding structural and thermal-hydraulic concerns with fuel channels. In order to deal with these concerns, accurate on-line measurements within the fuel channels are required.